1. Field of the Invention
The present invention relates to a method of strengthening geological formations in underground workings or mines, particularly in coal mines, by means of reaction mixtures based on organic polyisocyanates and polyhydroxyl compounds which react to form polyurethanes, in which special monofunctional alcohols are used as additives to regulate the foaming process.
2. Description of the Prior Art
The strengthening of geological formations underground, particularly in coal mining, by means of foamable polyurethanes which harden in the formation is already known (see e.g. the Journals Gluckauf (1968), pages 666 to 670, Gluckauf (1977), pages 707 to 711 and Bergbau (1977), pages 124 to 129, and DE-PS Nos. 1,129,894, 1,758,185, 1,784,458, 2,436,029, 2,623,464 or 3,139,395). Such mixtures which react to produce polyurethanes are also used to seal off structures against water and/or gases.
This state of the art process is generally carried out as follows: the two reaction components, the polyisocyanate and polyalcohol (polyhydroxyl component), are transported separately to a bore hole where they are continuously combined in a static mixer and forced through the bore hole into the rock formation where they foam up and harden.
The polyhydroxyl compounds used in this state of the art process are preferably based on mixtures of polyether polyols and castor oil (see e.g. Gluckauf (1968), pages 666 to 670). Fatty acid derivatives containing hydroxyl groups, in particular castor oil, increase tackiness and strength of adherence as well as flexibility of the PU mixtures. At the same time, these polyols function as defoamers, thereby, suppressing excessive foaming up of the polyurethane resins which would weaken the mechanical strength properties. The castor oil is normally used in quantities of about 1 to 50% by weight, preferably about 5 to 20% by weight, based on the polyol mixture. It therefore constitutes a considerable cost factor, especially since the price of this natural product is subject to wide fluctuations and is on the whole considerably higher than that of other polyols. In addition, being a naturally occurring product, castor oil fluctuates widely in quality. Therefore, there was a need for a better solution which would be free from these disadvantages, while preserving the advantageous effects mentioned above.
It has now surprisingly been found that by using small quantities of monofunctional alcohols, it is possible not only to avoid the disadvantages described above but also to improve the properties of the polyurethane in use.